"^"^ocSe;,  i9iTl       ^^^^      "^^^^y  of  Formaldehyde.  .459 
hyde  solution  there  might  be  an  error  of  ±  0.05  c.c.  or  ±  0.2  per 
cent.,  might  there  not  also'  be  a  similar  error  when  the  same  measur- 
ing instrument  will  be  used  for  measuring  out  the  25  c.c.  of  twice 
normal  NaOH  (according  to  Smith)  in  the  case  of  the  hydrogen 
peroxide  method?  Besides,  in  taking  25  c.c.  of  the  100  c.c.  of  the 
diluted  formaldehyde  solution  it  is  not  necessary  that  the  measuring- 
instrument  used  should  measure  out  exactly  25  c.c.  but  only  that  the 
volume  so  measured  out  be  one-fourth  of  the  total,  and  whether 
this  is  the  case  can  be  readily  and  conveniently  ascertained  by  using 
the  same  measuring  instrument  for  filling  the  empty  100  c.c.  flask 
with  water  and  seeing  whether  four  fillings  will  fill  it  exactly  to  the 
mark.  This  will  prove  the  accuracy  of  the  instrument  for  this  pur- 
pose or  enable  the  operator  to  apply  the  proper  correction.  It  would 
seem  therefore  that  if  the  potassium  cyanide  method  is  an  excellent 
method  for  determining  formaldehyde  in  dilute  solutions  it  may  be 
used  with  equal  advantage  in  the  case  of  strong  solutions  by  suitably 
diluting  the  latter  with  water. 
That  such  is  really  the  case  may  be  seen  from  Smith's  own 
results.  Using  the  potassium  cyanide  method  for  strong  formalde- 
hyde solutions,  in  which  case  all  solutions  containing  more  than 
one  per  cent,  were  diluted,"  the  three  results  given  in  the  case  of 
the  37  per  cent,  formaldehyde  solution  are:  37.12,  37.07,  and  37.18 
per  cent.  Likewise,  the  results  of  Williams,  when  using  the  potas- 
sium cyanide  method  for  determining  the  formaldehyde  in  the  con- 
centrated solution,  show  that  very  concordant  results  are  obtained  by 
this  method.  Thus  six  determinations  yielded  the  following  per- 
centages:  35.15,  35.06,  35.23,  35.01,  35.21,  35.07,  or  a  maximum 
difiference  from  the  average  (35.12)  of  only  o.ii  per  cent.  When 
using  the  hydrogen  peroxide  method,  four  determinations  yielded 
the  following  percentages :  35.82,  35.92,  35.74  and  35.78,  or  a  maxi- 
mum difference  from  the  average  (35.81)  of  o.ii  per  cent.  It  is 
thus  seen  that  the  potassium  cyanide  method  can  be  applied  to  strong 
formaldehyde  solutions  by  suitably  diluting  them,  the  results  thus 
obtained  being  equally  concordant  as  those  obtained  by  the  hydrogen 
peroxide  method.  It  therefore  remains  only  to  clear  up  the  point 
as  to  whether  or  not  the  lower  results  ol^tained  by  the  potassium 
cyanide  method  as  compared  with  those  obtained  by  the  hydrogen 
peroxide  method  may  be  due  to  possible  incompleteness  of  the  reac- 
tion between  the  KCN  and  the  formaldehyde. 
If  we  will  suppose  tliat  the  reaction  between  the  KCN  and  the 
